1. Field of the Invention
This invention relates broadly to radio communication systems. More particularly, this invention relates to spread spectrum cellular communication systems, such as 1xEV-DO systems, that employ a beacon signal (e.g., a pseudo-noise digital sequence in 1xEV-DO systems) as part of the forward communication link within the system.
2. State of the Art
Current spread spectrum cellular wireless technologies, such as CDMA2000 (also known as IS-2000), have features that significantly improve voice capacity and data services. However, such technologies are not fully optimized for high speed IP traffic. For example, the highest transmit rate on the forward link is 307.2 kbps using the 1X spreading version (1.25 MHz). Thus, a new cellular wireless technology, known as IS-856 or 1xEV-DO, was developed to provide efficient high rate packet data services without the constraints of supporting legacy modes of operation.
In 1xEV-DO systems, a network of Access Points (which are sometimes referred to as servers) provide high rate data access on a wireless CDMA channel to a collection of static or mobile Access Terminals (which are sometimes referred to as user terminals). The Access Points are analogous to base stations in IS-95 systems and are sometimes referred to as such, and the Access Terminals are analogous to mobile units in IS-95 systems and are sometimes referred to as such.
The information is transmitted from the Access Points to the Access Terminals over a forward link that is organized with a frame structure shown in FIG. 1. The total frame length is 26.67 ms and each frame is divided equally into sixteen 1.666 ms timeslots each carrying 2048 chips (1.2288 Mcps*1.666 ms). Each slot is further divided into two half-slots, each of which contains a pilot burst as shown in FIGS. 2A and 2B. Each pilot burst has a duration of 96 chips, and is centered at the midpoint of the half slot. Within each slot, a Pilot channel (which is realized by the pilot bursts), a Forward Medium Access Control (MAC) channel, and a Traffic/Control channels are time multiplexed. There are two different structures for the 1xEV-DO forward link time slot as illustrates in FIGS. 2A and 2B: one for an active slot where data transmission occurs (FIG. 2A) and one for the idle slot when there is no transmission of data to any user (FIG. 2B).
The time-division-multiplexed channels are transmitted at maximum power of the sector, thereby eliminating power sharing amongst active users. During data transmission, data is directed to only one Access Terminal at a time using the full power of the Access Point to allow the highest possible data rate for that one user.
The MAC channel consists of two subchannels: the Reverse Power Control (RPC) channel and the Reverse Activity (RA) channel. The Traffic/Control channels carry user data and control data between the Access Point and the Access Terminal. Details of the data carried in these channels are set forth in 3GPP2 C.0024 Version 2.0, “CDMA2000 High Rate Packet Data Air Interface Specification,” herein incorporated by reference in its entirety.
1xEV-DO systems employ a burst pilot, which is optimal for bursty packet data services. The burst pilot is transmitted on a separate code channel as in IS-95 and also it is punctured into the forward link waveform at pre-determined intervals as shown in FIG. 3. The burst pilot is transmitted at the maximum power that the cell is enabled to transmit. Using the full power of the cell for the pilot provides the highest possible Signal-To-Noise Ratio (SNR) so that an accurate channel estimation can be obtained quickly, even during dynamic channel conditions.
The burst pilots are sequences of 96 chips in length that are derived from a reference pseudo-noise (PN) sequence of 32,768 chips in length. The reference PN sequence is punctured at predetermined 96 chip intervals to select corresponding 96 chip subsequences (or chunks) therein. The predetermined intervals are offset by 64 chips such that there exists 512 (32,768/64) possible burst pilot sequences. Each of these 512 burst pilot sequences is identified by an index in the range from 0 to 511. One of the 512 possible burst pilot sequences is assigned to each Access Point in the network. This burst pilot sequence allows the Access Terminals to identify the Access Point and to acquire timing for initial acquisition, phase recovery, timing recovery, and maximal-ratio combining. It also provides a means for predicting the receive signal strength for the purposes of forward data rate control (DRC).
The synchronization of the Access Points of the 1xEV-DO system is achieved by the use of Global Positioning System (GPS) receivers at each Access Point location. Aided by appropriate stable clock generators, these GPS receivers supply accurate timing information to the Access Point.
Pilot pollution occurs within the coverage area of the 1xEV-DO system when numerous pilot signals are received with relatively equal signal strength. Such pilot pollution is detrimental because it may cause dropped data transfers and decreased capacity. Thus, it is advantageous to optimize the allocation of the burst pilot sequences over the Access Points of the network in order to minimize such pilot pollution.
Such optimization is typically accomplished by drive-testing the intended coverage area of the 1xEV-DO cellular system with a pilot scanner that detects the received pilot signals and measures/records the signal strengths of the detected piloted signals at various locations within the intended coverage area of the 1xEV-DO cellular system. The pilot scanner requires access to an accurate clock source. Typically, the Global Positioning System (GPS) is used as the clock source. As such, these devices typically have GPS receivers.
In 1xEV-DO systems, there is a general assumption that all Access Points must limit their power so that burst pilot sequences will be received by the Access Terminals within a maximum delay of 64 chips due to signal path propagation. However, if the network is tuned so that the burst pilot sequences can be received at further distances (or if the Access Points lose synchronization to the system clock), it is possible to receive burst pilot sequences in a chip delay greater than 64 chips.
In current 1xEV-DO pilot scanning systems, it is possible to employ a search window with a length greater than 64 chips to detect these situations. However, the extended search window introduces an ambiguity in the detection of such burst pilot sequences. More particularly, assume that the Access Point is transmitting a burst pilot sequence whose index is 256 and there is no signal delay due to propagation. When the pilot scanner searches for this burst pilot sequence, it will find burst pilot sequence 256 at zero chip delay and will also find burst pilot sequence 255 at 64 chip delay. This occurs due to the fact that in 1xEV-DO systems, two adjacent pilot signals have a 32 chip overlap. This ambiguity is commonly resolved by L3 message detection and decoding wherein the index of the two pilot signals is recovered and the duplicative pilot signal (in the example above, the burst pilot sequence 255 at 64 chip delay) is ignored.
Disadvantageously, such L3 message detection and decoding is complex and costly to develop and implement. Moreover, the signal processing operations required for such L3 detection and decoding add to the processing time of the scanner and ultimately slows down the scan speed. Thus, there remains a need to provide an improved apparatus and methodology for evaluating signal propagation and coverage, including burst pilot measurements, in 1xEV-DO systems and the like.